In a fuel cell system used as a power source of a vehicle, a fuel cell stack is often used. The fuel cell stack is obtained in such a manner that a fuel cell structure having an oxidant electrode and a fuel electrode provided as a pair interposing a solid polymer electrolyte film therebetween is sandwiched by separators, and a plurality of the fuel cell structures are laminated on each other. Electric power generated from the fuel cell stack is charged to a battery and then used to drive a motor in the vehicle.
With respect to a fuel cell system, a control, which is performed in such a manner that a charge level of a battery (SOC) is set high at the time of low speed driving to secure driving force of a motor and to secure electric power capable of being outputted from the battery, is disclosed in the gazette of Japanese Patent Laid-Open No. 2000-92610.
Also, with respect to a conventional fuel cell system, for example, a method is disclosed in the gazette of Japanese Patent Laid-Open No. 10(1998) -271706. In this method, a state is detected where a main load, such as a motor installed in a vehicle, is almost zero. As a result of the detection, when a judgment for the detected state tells that a battery is to be charged, the battery is charged. According to this method, in the conventional fuel cell system, it has been possible to supply sufficient electric power at all times, even when the electric power required by the load changes by a large amount.